1. Field of the Invention
The invention relates to a peripheral object detection apparatus that is installed in a vehicle to detect a peripheral object obstructing travel by a vehicle, and a peripheral object detection method.
2. Description of Related Art
In a collision avoidance assist system such as a pre-crash system, an object (a vehicle or the like) obstructing travel by a vehicle must be detected with a high degree of precision. A radar such as a millimeter wave radar is used for the detection, and an object causing an obstruction is differentiated from an object positioned below the vehicle and therefore not obstructing travel (an object over which the vehicle can pass) on the basis of a reflection intensity obtained by the radar. In a vehicle radar apparatus described in Japanese Patent Application Publication No. 2010-204033 (JP-2010-204033 A), for example, when a reflection wave is detected by a radar, a difference between a detected waveform and a reference waveform is calculated. When an intensity of a resulting difference signal equals or exceeds a threshold, the reflection wave is determined to be from an unneeded object such as a manhole or a metal joint on the road.
An obstacle over which the vehicle cannot pass has at least a certain height from a ground surface. Therefore, paths along which electromagnetic waves (millimeter waves or the like) transmitted by the radar return to the radar after being reflected by the obstacle include a path along which the electromagnetic waves return directly from the obstacle and a path along which the electromagnetic waves reflected by the obstacle return indirectly after being further reflected by the ground surface. When the reflection waves, are received by the radar, the reflection intensity basically increases as a relative distance to the obstacle decreases. When reflection waves traveling in a multipath environment are received by the radar, however, the reflection waves traveling on the respective paths, which have varying distances, interfere with each other so as to be amplified or canceled out, and as a result, a plurality of peak portions and trough portions having large variation amounts are formed in a variation pattern of the reflection intensity relative to the relative distance (see FIG. 2C).
When a low object U1 (a steel plate used in construction work or the like) over which the vehicle can pass is disposed correctly on the ground surface, as shown in FIG. 5A, the low object U1 substantially does not project from the ground surface. Therefore, the only path along which the electromagnetic waves transmitted by the radar return to the radar after being reflected by the low object U1 is a path along which the electromagnetic waves return directly from the low object U1. When reflection waves traveling along a single path are received by the radar, the reflection intensity simply increases as the relative distance to the low object U1 decreases, and therefore the peak portions and trough portions formed in relation to the reflection intensity from the obstacle do not occur in the variation pattern of the reflection intensity relative to the relative distance (see FIG. 2A). Instead, a single large peak portion is formed in the variation pattern of the reflection intensity relative to the relative distance. In a conventional technique, low objects are differentiated from obstacles by focusing on the number of peak portions and trough portions in the reflection intensity.
In a case where a low object U2 overlaps with another low object, however, as shown in FIG. 5B, the vehicle is capable of passing over the low object U2, but the low object U2 projects slightly from the ground surface. Therefore, the paths along which the electromagnetic waves transmitted by the radar return to the radar after being reflected by the low object U2 include both the path along which the electromagnetic waves return directly from the low object U2 and paths along which the electromagnetic waves reflected by the low object U2 return indirectly after being further reflected by the ground surface. Far fewer paths along which the electromagnetic waves return after being reflected by the ground surface exist than in the case of the obstacle described above. Therefore, when reflection waves traveling in a multipath environment are received by the radar in this case, although as a whole the reflection intensity increases as the relative distance to the low object U2 decreases, peak portions and trough portions having small variation amounts are formed in the variation pattern of the reflection intensity relative to the relative distance (see FIG. 2B). This reflection intensity variation pattern is observed not only when a plurality of low objects overlap, but also when a low object is suspended above the ground surface and when the low object itself is thick. When this reflection intensity variation pattern is detected using the aforesaid method of focusing on the number of peak portions and trough portions in the reflection intensity, it may be impossible to differentiate correctly between a low object and an obstacle. When a low object over which the vehicle can pass safely is erroneously detected as an obstacle, unnecessary support may be provided by the collision avoidance support system.